This invention relates to a process for the isolation and recovery of .beta.-sitosterol (I) substantially free from .alpha.-sitosterol (II).
The steroids in general form an important group of the modern drugs. One of these steroids is .beta.-sitosterol, which is a lipotropic agent. .beta.-sitosterol is even more important as a starting material in the production of other steroids. .alpha.-sitosterol is a harmful agent in the conversion of .beta.-sitosterol to other steroid derivatives. It is therefor desirable to produce .beta.-sitosterol free of .alpha.-sitosterol.
The present invention is a process for obtaining .beta.-sitosterol which is substantially free of .alpha.-sitosterol from starting materials containing both .alpha. and .beta. sitosterol, such as the unsaponifiable fraction obtained as a by-product of soap manufacturing from the crude soap skimmings of the sulfate pulp process using as raw material both pinewood and hardwood, especially birch. The .beta.-sitosterol isolated according to the invention is pure enough for the use as starting material in the preparation of steroid intermediates as well as pharmaceutical .beta.-sitosterol. ##STR1##
Although many processes for the separation of sterols from various sources are known, not many of these processes deal with the problems inherent in the separation of .beta.-sitosterol from the neutral fraction obtained in the sulfate pulp processing of pinewood and hardwood. More particularly, prior processes were not generally concerned with obtaining .beta.-sitosterol which is substantially free of .alpha.-sitosterol, from a mixture thereof.
U.S. Pat. No. 2,835,682, for example, concerns the recovery of sterols from sterol-containing materials in general. The method disclosed in this patent comprises fractionating a sterol containing mixture in a liquified, normally gaseous hydrocarbon, e.g. propane, to give a sterol-enriched fraction. The sterol-enriched fraction is then saponified in alcoholic alkali solution whereafter the sterols are crystallized by adding water, and cooling.
U.S. Pat. No. 2,866,797 shows the separation of sterols from unsaponifiables obtained from vegetable oils, tall oils, sugar cane oil and the like, by extraction and crystallization. The unsaponifiable fraction is extracted with ethylene dichloride, and small amounts of water and methanol are added to precipitate the sterols.
A more recent publication, U.S. Pat. No. 3,691,211, teaches a process for preparing sterols from plant sources, especially tall oil pitch, by extraction in a water-alcohol-hydrocarbon mixture, followed by saponification and subsequent recrystallization and leaching. The starting materials for this process are different than those contemplated for use in the present invention and the problems to be solved are different. Thus, although a good yield is obtained, it is not surprising that the process itself is not comparable to that of the present invention.
Another purification method is generally described in Chemical Abstracts, Vol. 81 (1974) 51409 v for purifying crude phytosterol derived from sulfate soap, to .beta.-sitosterol. The process comprises dissolving in ligroin at 70.degree.-75.degree. C. and washing with water at 65.degree.-70.degree. C. The solution is then cooled to give 90.4 percent pure .beta.-sitosterol, the yield being 69.5 percent.
The process disclosed in U.S. Pat. No. 4,044,031 is for the separation of sterols from e.g. the same neutral fraction as in the present invention. The process of U.S. Pat. No. 4,044,031 consists of dissolving the neutral fraction in a water-immiscible solvent, extracting the solution with a hydrophilic phase containing small amounts of water, and recovering sterols from the hydrophilic phase. This process, which utilizes extraction with two solvent phases, can be carried out continuously utilizing a counter-current extraction process.
As compared with all above mentioned processes, the process of the present invention is simpler and gives a better result. The present invention process successfully obtains .beta.-sitosterol which is substantially free of .alpha.-sitosterol, and on a commercial scale.
No good process for the separation of .alpha.- and .beta.-sitosterol is known. According to U.S. Pat. No. 2,573,265, steroids with a 3.beta.-OH-group and a C.sub.5-6 double bond, as for instance .beta.-sitosterol, form acid addition products with HCLO.sub.4 and HPF.sub.6, which thereafter can be removed from the other neutral products. In the publication Sci.Res. (Dakka, Pak.) 1969, 162, the separation of .alpha.- and .beta.-sitosterol chromatographically on aluminum oxide is described.
The present invention process is based on observations made during experiments with the purification of .beta.-sitosterol. It was found, that .alpha.-sitosterol reacts much more easily with acids than .beta.-sitosterol. By observing the reaction gas chromatographically and mass spectrometrically it was found, that .alpha.-sitosterol is rearranged in acid conditions so that the position of the double bond in the ring is changed, whereby many rearrangement products are formed, which have not been identified. If the acid treatment is continued for longer than the optimum time, the OH-group of .alpha.-sitosterol and at a later stage also the OH-group of .beta.-sitosterol are split off to give dehydration products. In addition to this, a substitution of the OH-group with an acid rest, e.g. chlorine, occurs. The solubilities of both the rearrangement products and the dehydration products differ so much from the solubility of the sterol components that they are easily removed by crystallization. By suitable adjustment of the conditions only rearrangement products are obtained.
According to the book L. F. Fieser and M. Fieser, "Steroids," Reinhold Publishing Corp., New York 159, pages 113 and 114, .DELTA..sup.8(13) -ergostenol is isomerized in the presence of hydrogen chloride in chloroform to .DELTA..sup.14 -ergostenol, and 5-hydroergosterol is isomerized under the same conditions to a mixture containing .DELTA..sup.8(14),22 - and .DELTA..sup.14,22 -ergostadienol. The reaction of .alpha.- and .beta.-sitosterol with acids has not, however, been studied before.
The rearrangement occurs only at the C.sub.7-8 double bond of .alpha.-sitosterol but not at the C.sub.5-6 double bond of .beta.-sitosterol. It is surprising that the reaction products can be removed from the mixture by a simple crystallization. Although rearrangements of this kind by steroids have been earlier described, it is surprising that the reaction can be utilized with such good result specifically for removing .alpha.-sitosterol from .beta.-sitosterol containing raw material.